Project Summary Endothelial progenitor cells (EPCs) are adult stem cells with roles in angiogenesis and endothelial function. Human and animal studies suggest that autologous stem cell therapy using an EPC enriched population of cells can be therapeutic in diseases such as hypertension, myocardial infarction, stroke, and diabetes. However, EPCs from diseased and aging hosts show decreases in number and function that are likely to compromise therapeutic efficacy, thus masking the potential of EPC based therapy. The objective of this study is to determine the mechanism of impaired angiogenic competency of EPCs in an animal model of EPC dysfunction. In the proposed animal model, the SS/MCWi (SS) rat, EPC competency and longevity is impaired in vivo but rescued in vitro. We have observed an increase in the natural killer (NK) cell receptor 2B4 (CD244) on the membrane of SS EPCs and an increase in killing of SS EPCs by SS NK cells. We hypothesize that increased expression of CD244 on the EPC membrane leads to increased NK cell mediated destruction of SS EPCs in vivo, which compromises their ability to induce angiogenesis. We will use an in vitro cytotoxicity assay and an in vivo model of hind-limb angiogenesis to test the effects of CD244 knockdown and NK cell depletion on NK cell mediated destruction and angiogenic competency of SS EPCs. We will also assess the physiological impact of the various observed CD244 transcript sequences in an in vitro cell culture system. Because we have previously shown that low-dose angiotensin II (AngII) restores angiogenesis in SS rats and angiogenic competency in SS bone marrow mononuclear cells, we will assess the effects of AngII on the angiogenic competency, CD244 expression, and NK cell mediated cytotoxicity of SS EPCs.